People smarter than all of us combined have interpreted the Hawaii gun laws regarding > 10rd mags. The reading of the law as written pretty much bans any rifle mag over 10 rds if a pistol that can accept that mag exists. Not "except if that pistol is banned and can't be legally owned here," and not "if you don't own a pistol the mag fits." If you buy a carbine rifle that accepts 17rd or 30rd Glock 9mm mags, it doesn't matter whether or not you own a 9mm Glock. The simple fact that the mag fits a pistol you can buy from a dealer is all that matters.
AR mags have existed in a "grayish area", due to the banning of AR pistols as manufactured. All factory AR pistols have detachable mags. While the most common method of making an AR pistol HI compliant is to epoxy/weld the mag into the magwell, making it impossible to insert a 20rd or 30rd AR mag, it's not the only method to become compliant. As a side note, once you make the mag nondetachable, the 10rd limit no longer applies, so you could weld a 30rd mag into the pistol and be compliant.
The real underlying difference is enforcement. Cops don't seem to care if you have >10rd mags for your AR rifles. So, they don't bother arresting anyone for it. Since it is such an ambiguous statute when you factor in whether or not the Assault Pistol is illegal to possess, not a home-built AR pistol, they prefer to ignore it.
You can make ANY rifle into a pistol at home. I think the lawmakers were more concerned with factory manufactured pistols. Otherwise, why even tie this law to pistols at all? If any receiver can become a pistol, then all rifle mags over 10rds are prohibited.
Handguns have always been involved in crimes more than rifles. So, this 10rd limit was really about handguns. The rifle/handgun tie-in was so you can't possess >10rd mags legally for a rifle while also owning a pistol that the mag can be used in. But, as usual, the lawmakers created a mess rather than detailing what the intent was. I think it would have been better to do as some people (OG) believe already applies. Make possession of the pistol that accepts your rifle mags of >10rds illegal. If all you have that fits your >10rd mag is a rifle, then no problem.
As written, the statute is ambiguous and confusing. We all assume nobody is going to be charged with >10rd mags unless the mags are used in common factory-built pistols, and it's added as an additional charge to other worse ones.
I'm not a lawyer. This is my summation of the AR Mag conundrum. Take it with a grain of salt.
I personally only take 10rd or smaller capacity mags to the range. Can't load more than 5 rds on the rifle range anyway, and the shorter mag doesn't get in the way as much when shooting at the bench.
As for buying online, it's hit or miss. Some don't want to get involved in potential legal problems. Others feel it's on the buyer to know their laws. I've had buyers who shipped 30rd mags for 10 years suddenly stop. It's just how things are.